The field of the present invention is control of air driven diaphragm pumps.
Pumps having double diaphragms driven by compressed air directed through an actuator valve are well known. Reference is made to U.S. Pat. Nos. 5,169,296; 4,247,264; 294,946; 294,947; and 275,858, all issued to James K. Wilden, the disclosures of which are incorporated herein by reference. An actuator valve operated on a feedback control system is disclosed in U.S. Pat. No. 3,071,118 issued to James K. Wilden, the disclosure of which is also incorporated herein by reference. This feedback control system has been employed with the double diaphragm pumps illustrated in the other patents.
Such pumps include an air chamber housing having a center section and two concave discs facing outwardly from the center section. Opposing the two concave discs are pump chamber housings. The pump chamber housings are coupled with an inlet manifold and an outlet manifold through ball check valves positioned in the inlet passageways and outlet passageways from and to the inlet and outlet manifolds, respectively. Diaphragms extend outwardly to mating surfaces between the concave discs and the pump chamber housings. The diaphragms with the concave discs and with the pump chamber housings each define an air chamber and a pump chamber to either side thereof. At the centers thereof, the diaphragms are fixed to a control shaft which slidably extends through the air chamber housing.
Actuator valves associated with such pumps have included feedback control mechanisms including a valve piston and airways on the control shaft attached to the diaphragms. Air pressure is alternately generated in each air chamber according to control shaft location, driving the diaphragms back and forth. In turn, the pump chambers alternately expand and contract to pump material therethrough. Such pumps are capable of pumping a wide variety of materials of widely varying consistency.
FIGS. 1 and 2 illustrate a previously designed control rod or shaft and associated bushing, respectively. The shaft PA1 has a center portion having a waist PA2 of reduced cross-sectional dimension in the otherwise cylindrical shaft PA1. Axial slots are equiangularly spaced about the waist PA2 to provide added axial air flow. The associated bushing PA3 has three annular channels to either side of a central portion. The innermost and outermost channels PA4 and PA5 of each set of three receive O-rings to act as annular seals between the bushing PA3 and the shaft PA1 in order that flow may be controlled between the central annular channels PA6 and vent passages PA7.
The valving mechanism provided by the shaft PA1 and the bushing PA3 cooperates with a control valve to alternately vent either end of a shuttle piston at the ends of the stroke of the shaft PA1. The venting occurs when the waist portion PA2 spans alternately the two innermost channels PA4 to expose the central annular channels PA6 to the vent passages PA7. The waist portion PA2 provides both an axial passage capable of spanning the aforementioned seals and a circular manifold for venting annular air flow across the seal to the vent passages PA7 at either side. This arrangement has long been employed because of the need to rapidly vent the appropriate passage of the control valve. By using the waist PA2 with a conical transition surface, the O-ring associated with the seal is caused by relative pressure differential across the seal to deform and remain in contact with the surface of the conical portion of the waist PA2 until it finally overcomes that pressure differential and pops back to an undeformed state. In doing so, it instantaneously opens a substantial cross-sectional area between the O-ring and the conical portion of the waist PA2. This action results in the area between the O-ring and the waist portion PA2 not being the area of greatest flow resistance at the moment of opening. The vented air may then pass around the waist PA2 to a vent passage PA7. The venting of one passage from the associated control valve causes the shuttle piston therein to be drawn to the vented end. It has been found necessary to insure rapid movement of that piston so as to prevent a condition of stall. This configuration of the shaft PA2 in other than a fully cylindrical shape has resulted in excessive O-ring wear and the need for O-rings of high shore hardness.